114 EVOLUTION AND GENETICS 



tains copper and iron, the former in relatively large quantities. 

 Haemoglobin, the iron compound which gives the red color to 

 vertebrate blood and serves as a vehicle for transporting oxygen 

 in the body is also present in mollusca to a limited degree. Thus 

 the blood of the three classes is similar as a conveyor of absorbed 

 food and blood cells, but in only two cases is it a conveyor of oxy- 

 gen, and in these two it accomplishes its respiratory function by 

 means of different substances. 



Metabolism. For a more generally applicable case we have only 

 to refer to the several metabolic processes mentioned in Chap- 

 ter VI. All animals must have energy. The response of the organism 

 to its environment invariably involves the controlled release of 

 energy in some form. Transformation or release of energy is nob 

 limited in the physical world to any one process, and the fact that 

 the iron bacteria and sulphur bacteria secure their energy through 

 two processes while most organisms carry on a third entirely 

 different process is sufficient evidence that living organisms are 

 not necessarily limited to one source of energy. It is very strongly 

 suggestive of similarity of function through similarity of origin. 



The Cause and Process of Change. In these examples organ- 

 isms are seen to differ in degrees corresponding to the remoteness 

 of their relationship. If all species were wholly independent in 

 origin, we might expect a variety of structures and functions no 

 less than the number of species involved. That similar results 

 would be accomplished in many cases l)ecause of similar needs we 

 cannot doubt, for examples are before us, but it is equally im- 

 possible to believe that these independent species would follow 

 in their individual development unnecessarily tortuous paths, or 

 that they would produce even temporarily structures which were 

 of no use. 



Factors in Existence. The questions naturally occur, why 

 should these differences have come about, and how? Evolution- 

 ists have attempted to answer these questions, although in these 

 attempts we still find the purely theoretical aspects of the subject. 

 The interpretation of relationships leaves no doubt of the reality 

 of evolution, but the exact forces through which it is expressed are 

 not yet proved. These problems must be considered in detail, but 

 to establish completely the evidences of evolution it is necessary 

 to consider in brief the possibility of such changes. 



Returning to the analogy of individual relationship, the three 



